The exemplary embodiments described herein relate to methods for producing polyolefin polymers.
Typical polyolefin polymerization reactions in a fluidized bed gas phase reactor employ a continuous cycle. In one part of the cycle, a cycling gas stream (sometimes referred to as a recycle stream or fluidizing medium) is heated in the reactor by the heat of polymerization. This heat is removed from the recycle stream in another part of the cycle by a cooling system external to the reactor. Generally, in a gas fluidized bed process for producing a polyolefin product, the recycle stream is a primarily gaseous stream containing an olefin monomer, optionally hydrogen, and optionally at least one comonomer that is continuously cycled through the fluidized bed in the presence of a catalyst under reactive conditions. The recycle stream is withdrawn from the fluidized bed and (after cooling) is recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and fresh olefin monomer, the optional hydrogen, and the optional comonomers are added to replace any that has polymerized or been entrained in the polyolefin product stream.
In some conventional polymerization reactions, a fluidized bed gas phase reactor system operates in a “condensed mode” (e.g., as described in International Patent App. Pub. No. WO 2007/030915) in which the recycle stream is cooled to a temperature below the dew point in the reactor. Typically, this is accomplished by including an ICA in an appropriate concentration and controlling the recycle stream temperatures so as to condense the ICA portion of the recycle gas stream. Generally, condensed mode production of polyolefins facilitates a greater production rate of the polyolefin.
The polyolefin from the reactor is typically in the form of granules that may be degassed and then extruded into pellets that are sold to customers. The pellets may then be processed into various articles including thin films for food packaging, resealable baggies, and the like. Such articles preferably have very high clarity and few defects. One source of defects that also impact clarity is small gels (i.e., gel particles having a diameter of 201 microns to 600 microns). It is believed that the small gels arise from portions of the polyolefin that have not completely melted and blended with the surrounding polyolefin.